Sensors

ImportantConcepts Mostremotesensinginstruments(sensors)aredesignedtomeasurephotons.

Thefundamentalprincipleunderlyingsensoroperationcentersonwhathappensinacriticalcomponent thedetector.

Thisistheconceptofthephotoelectriceffect (foundbyAlbertEinstein)

Thissaysthattherewillbeanemissionofnegativeparticles(electrons)whenanegativelychargedplateofsomeappropriatelightsensitivematerialissubjectedtoabeamofphotons.

Theelectronscanthenbemadetoflowasacurrentfromtheplate,arecollected,andthen

countedasasignal.

Akeypoint:Themagnitudeoftheelectriccurrentproduced(numberofphotoelectronsperunittime)isdirectlyproportionaltothelightintensity.

Thus,changesintheelectriccurrentcanbeusedtomeasurechangesinthephotons(numbers; intensity)thatstriketheplate(detector)duringagiventimeinterval(integrationtime).

Thekineticenergyofthereleasedphotoelectronsvarieswithfrequency(orwavelength)oftheimpingingradiation.

But,differentmaterialsundergophotoelectriceffectreleaseofelectronsoverdifferentwavelengthintervals;eachhasathresholdwavelengthatwhichthephenomenonbeginsandalongerwavelengthatwhichitceases.

Now,withthisprincipleestablishedasthebasisfortheoperationofmostremotesensors,letussummarizeseveralmainideasastosensortypes(classification)intwodiagrams:

Thisisafunctionaltreatmentofseveralclassesofsensors,plottedasatrianglediagram,inwhichthecornermembersaredeterminedbytheprincipalparametermeasured:Spectral;Spatial;Intensity.

Radiometer isageneraltermforanyinstrumentthatquantitativelymeasurestheEMradiationinsomeintervaloftheEMspectrum.

Whentheradiationislightfromthenarrowspectralbandincludingthevisible,thetermphotometer canbesubstituted.Ifthesensorincludesacomponent,suchasaprismordiffractiongrating,thatcanbreakradiationextendingoverapartofthespectrumintodiscretewavelengthsanddisperse(orseparate)thematdifferentanglestoanarrayofdetectors,itiscalledaspectrometer.

Thetermspectroradiometer isreservedforsensorsthatcollectthedispersedradiationinbands ratherthandiscretewavelengths.Mostair/spacesensorsarespectroradiometers.

Ternarydiagramoftheclassesofsensors

SirIsaacNewtondiscoveredthatwhitelightcouldbedispersedintoitsspectralcomponentsbypassingitthroughaprism.

Classificationofsensors

Thesecondcoversawiderarrayofsensortypes:

Thetwobroadestclassesofsensors:

Passive (energyleadingtoradiationreceivedcomesfromanexternalsource,e.g.,theSun);Passivesensorsoperateinthevisiblewaveband,thermalIRandmicrowavedomains.

Active (energygeneratedfromwithinthesensorsystem,beamedoutward,andthefractionreturnedismeasured).Activesensorsonsatellitealloperateinthemicrowave.

NotethatactivesensorsoperatinginthevisiblewavebandareknownasLIDAR.

Sensorscanbe:

Nonimaging (measurestheradiationreceivedfromallpointsinthesensedtarget,integratesthis,andreportstheresultasanelectricalsignalstrengthorsomeotherquantitativeattribute,suchasradiance)

Imaging (theelectronsreleasedareusedtoexciteorionizeasubstancelikesilver(Ag)infilmortodriveanimageproducingdevicelikeaTVorcomputermonitororacathoderaytubeoroscilloscopeorabatteryofelectronicdetectors);sincetheradiationisrelatedtospecificpointsinthetarget,theendresultisanimageorarasterdisplay.

Passivesensors Wavelength Information

Visible/NearInfraredwavelengthradiometers

400nm 1m Solarradiationreflectedbyocean/land

ThermalInfrared(TIR)radiometers

about10m Thermalemissionoftheocean/land

Microwaveradiometers 1.5 300mm Thermalemissionoftheocean/landinthe

microwave

Activedevices

Altimeters 3 30GHz Seasurfacetopography

Scatterometers 3 30GHz Seasurfaceroughness

Syntheticapertureradars 3 30GHz Sea/landsurfaceroughnessandmovement

Natureofspacebornesensorsandtheirapplicability

Visible/NearInfraredRemoteSensing

Theobservationmethodtoacquirevisiblelightandnearinfraredraysofsunlightreflectedbyobjectsontheground.

Byexaminingthestrengthofreflection(atdifferentwavelengths),wecanunderstandaconditionsoflandsurface,e.g.,distributionofplants,urbanareas,rivers,seas.

Duringperiodofdarkness,Thismethodcannotmakeobservation.Also,cloudsblockthereflectedsunlight,sothismethodcannotobserveareasunderclouds.

ThermalInfraredRemoteSensing

Theobservationmethodtoacquirethermalinfraredrays,whichisradiatedfromlandsurfaceheatedbysunlight.Alsoitcanobservethehightemperatureareas,suchasvolcanicactivitiesandfires.

Byexaminingthestrengthofradiation,wecanunderstandsurfacetemperaturesoflandandsea,andstatusofvolcanicactivitiesandforestfires.

Thismethodcanobserveatnightwhenthereisnocloud,butcannotmakeobservationundercloudyconditions.

MicrowaveRemoteSensing

Microwavesensorsreceivemicrowaves,whichislongerwavelengththanvisiblelightandinfraredrays,andobservationisnotaffectedbyday,nightorweather.

Therearetwotypesofobservationmethodsusingmicrowavesensor:activeandpassive.

ActivetypeThesensoraboardearthobservationsatelliteemitsmicrowavesandobservesmicrowavesreflectedbysea(orland)surface.Itissuitabletoobservelandandseaareas

PassivetypeThistypeobservesmicrowavesnaturallyradiatedfromlandsurface.Itissuitabletoobserveseasurfacetemperature,snowaccumulation,thicknessofice.

**Microwavealtimeterandscatterometerarenonimagingtypesofsensorsthatcomeunderactivesensors.Theirobservationsarenotaffectedbyday,nightorweather.

Schematicillustratingthedifferentremotesensingmethodsandclassesofsensorsusedinoceanremotesensing,alongwiththeirpotentialapplications

OceanSensorsandtheirapplicability

Past,presentandnearfutureoceansatellitemissions

Past,presentandnearfutureoceansatellitemissionscontd

Past,presentandnearfutureoceansatellitemissionscontd

Past,presentandnearfutureoceansatellitemissionscontd

Past,presentandnearfutureoceansatellitemissionscontd

PresentStatusSeriesofOperationalandR&Dsatellitesensorsforoceanographyhasbeenandwillbefunctioningmorethantenyears

AltimeterScatterometerSSTsensorIcesensorOceancolour Sensor

LandRemoteSensors

Bands with AVHRR

Sensor Resolution(m)SwathWidth

(km)Sensor

Channels SpectralBands(m)

LinearImagingSelfScanningSystemI(LISSI) 72 148

LISSI1LISSI2LISSI3LISSI4

0.450.52(blue)0.520.59(green)0.620.68(red)

0.770.86(nearIR)

LinearImagingSelfScanningSystemII(LISSII) 36 74

LISSII1LISSII2LISSII3LISSII4

0.450.52(blue)0.520.59(green)0.620.68(red)

0.770.86(nearIR)

LinearImagingSelfScanningSystemIII(LISSIII)

23

50

142

148

LISSIII2LISSIII3LISSIII4LISSIII5

0.520.59(green)0.620.68(red)

0.770.86(nearIR)1.551.70(midIR)

6 70 PAN 0.50.75

HighResolutionLinearImagingSelfScanningSystemIV(LISSIV) 5.8 24 70

LISSIV2LISSIV3LISSIV4

0.520.59(green)0.620.68(red)

0.770.86(nearIR)

WideFieldSensor(WiFS) 188 774WiFS1WiFS2

0.620.68(red)0.770.86(near IR)

AdvancedWideFieldSensor(AWiFS) 5670 370740

AWiFS1AWiFS2AWiFS3AWiFS4

0.520.59(green)0.620.68(red)

0.770.86(nearIR)1.551.70(midIR)

Indian Remote Sensing (IRS) Satellite Sensors

Multi-angle Imaging Spectroradiometer (MISR) Onboard Terra

Airbornehyperspectral

sensors:

1. AVIRIS2. CASI

Satellitehyperspectral

sensor:

Hyperion

Hyperspectral sensors:

The IFOV and FOV can be calculated using trigonometry

(d is the detector size, f is the focal length). The general unit of IFOV is milliradians (10-3 radians). 1= 0.01745 radians = 17.45 milliradians1 milliradian= 0.057

IFOV and FOV

Thetermscanning canbeappliedbothtomovementoftheentiresensorforlightgathering.

Twobroadcategoriesofmostscannersaredefinedbytheterms"opticalmechanical"and"opticalelectronic

(b)opticalmechanical containinganessentialmechanicalcomponent(e.g.,amovingmirror)thatparticipatesinscanningthescene

TheCrossTrackmodenormallyusesarotating(spinning)oroscillatingmirror(makingthesensoranopticalmechanicaldevice)tosweepthescenealongalinetraversingtheground.ThisissometimesreferredtoastheWhiskbroommodefromthevisionofsweepingatablesidetosidebyasmallhandheldbroom.

(c)opticalelectronic havingthesensedradiationmovedirectlythroughtheopticsontoalinearortwodimensionalarrayofdetectors(CCDs).

TheAlongTrack Scannerhasalineararrayofdetectorsorientednormaltoflightpath.TheIFOVofeachdetectorsweepsapathparallelwiththeflightdirection.Thistypeofscanningisalsoreferredtoaspushbroom scanning(fromthementalimageofcleaningafloorwithawidebroomthroughsuccessiveforwardsweeps).

Characterization of optical sensors (visible, NIR and TIR)